From 7e0f021a9aec35fd8e6725e87e3313b101d26f5e Mon Sep 17 00:00:00 2001 From: Tobias Klauser Date: Sun, 27 Jan 2008 11:37:44 +0100 Subject: Initial import (2.0.2-6) --- reference/C/SYNTAX/volatile.html | 97 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 97 insertions(+) create mode 100644 reference/C/SYNTAX/volatile.html (limited to 'reference/C/SYNTAX/volatile.html') diff --git a/reference/C/SYNTAX/volatile.html b/reference/C/SYNTAX/volatile.html new file mode 100644 index 0000000..1de9b13 --- /dev/null +++ b/reference/C/SYNTAX/volatile.html @@ -0,0 +1,97 @@ + + +The VOLATILE keyword. + + + +

The VOLATILE keyword.

+I have had several sugestions on how to describe volatile, +If you have any input please mail me. +

The volatile keyword acts as a +data type qualifier. +The qualifier alters the default why in which +the compiler handles the variable and does not attempt to optimize the +storage referenced by it. Martin Leslie +
+
volatile means the storage is likely to change at anytime +and be changed but something outside the control of the user program. +This means that if you reference the variable, +the program should always check the physical address (ie a mapped input fifo), +and not use it in a cashed way.Stephen Hunt +
+

Here is an example for the usage of the volatile keyword: +

+	/* Base address of the data input latch */
+
+	volatile unsigned char *baseAddr;
+
+	/* read parts of output latch */
+
+	lsb    = *handle->baseAddr;
+	middle = *handle->baseAddr;
+	msb    = *handle->baseAddr;
+

+Between reads the bytes are changed in the latch. +

+Without the volatile, the compiler optimises this to a single assignment: +

+	lsb = middle = msb = *handle->baseAddr;
+

+Tim Potter +

A volatile variable is for dynamic use. E.G. for data that is to +be passed to an I/O port Here is an example. + +
```
+	#define TTYPORT 0x17755U
+
+	volatile char *port17 = (char)*TTYPORT;
+	*port17 = 'o';
+	*port17 = 'N';
+
```
+Without the volatile modifier, the compiler would think that the +statement *port17 = 'o'; is redundant and would remove it +from the object code. The volatile statement prevents the compiler +optimisation.Alex Arzon. +

+ +

Examples:

No example:-( +
+

The VOLATILE keyword.

Examples:

See also: